Chevrolet TrailBlazer, Service Manual

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against the inner side of the drum. This action forces the bottoms of the two shoes to
contact the brake drum by rotating the entire assembly slightly (known as servo action).
When pressure within the wheel cylinder is relaxed, return springs pull the shoes back
away from the drum.
Most modern drum brakes are designed to self-adjust themselves during application when
the vehicle is moving in reverse. This motion causes both shoes to rotate very slightly with
the drum, rocking an adjusting lever, thereby causing rotation of the adjusting screw.
Some drum brake systems are designed to self-adjust during application whenever the
brakes are applied. This on-board adjustment system reduces the need for maintenance
adjustments and keeps both the brake function and pedal feel satisfactory.
Power Boosters
Virtually all modern vehicles use a vacuum assisted power brake system to multiply the
braking force and reduce pedal effort. Since vacuum is always available when the engine
is operating, the system is simple and efficient. A vacuum diaphragm is located on the
front of the master cylinder and assists the driver in applying the brakes, reducing both the
effort and travel he must put into moving the brake pedal.
The vacuum diaphragm housing is normally connected to the intake manifold by a vacuum
hose. A check valve is placed at the point where the hose enters the diaphragm housing,
so that during periods of low manifold vacuum brakes assist will not be lost.
Depressing the brake pedal closes off the vacuum source and allows atmospheric
pressure to enter on one side of the diaphragm. This causes the master cylinder pistons to
move and apply the brakes. When the brake pedal is released, vacuum is applied to both
sides of the diaphragm and springs return the diaphragm and master cylinder pistons to
the released position.
If the vacuum supply fails, the brake pedal rod will contact the end of the master cylinder
actuator rod and the system will apply the brakes without any power assistance. The driver
will notice that much higher pedal effort is needed to stop the car and that the pedal feels
harder than usual.
Brake Hoses And Pipes
Always use double walled steel brake lines when replacing rusted or damaged brake lines.
The use of any other tubing is not approved and may cause brake failure. Carefully route
and retain replacement brake lines. Always use the correct fasteners and the original
location for replacement brake pipes. Failure to properly route and retain brake lines may
cause damage to the brake lines and cause brake system failure, resulting in possible
personal injury.
The steel brake lines use what is called an I.S.O. flare. This is different from the flare used
on American vehicles for many years. If a brake line is damaged and requires
replacement, you must obtain a replacement brake line with the correct flare. Most
automotive parts stores stock straight lengths of steel brake line, already flared, with the
correct fittings installed. Brake line replacement requires obtaining a piece of brake line as
close in length to the original as possible. The brake line must be of the correct diameter